1. Field of the Invention
The present invention relates to a light emitting element and an azole compound used therefore, and, more particularly, to a light emitting element which can be utilized in a wide range of fields such as back lights, flat panel displays, illuminating light sources, display elements, electrophotographs, organic semiconductor lasers, recording light sources, exposure light sources, read light sources, beacons, signboards, interiors and optical communication devices.
2. Description of Related Art
Among various light emitting elements which are being studied and developed today, organic electroluminescence (EL) elements have been animatedly studied and developed in recent years because these organic electroluminescence elements enable high luminance emission to be obtained even when driven at low voltages. An organic EL element is generally structured of a light emitting layer and a pair of counter electrodes between which the light emitting layer is sandwiched, and makes use of emission from an exciton produced by recombination of an electron injected from a cathode with a hole injected from an anode in the light emitting layer.
As organic EL elements which emit high illuminance light at low voltages at present, those having a multilayer structure as revealed by Tang et al. (Applied Physics Letters, vol. 51, page 913, 1987) are known. This element can emit high illuminance green light by lamination of a material which doubles as an electron transporting material and a light emitting material with a hole transporting material, wherein the illuminance reaches several thousands cd/m2 at a d.c. voltage of 6 to 7 V. However, further improvement in the illuminance and development of more highly efficient light emitting elements are desired, taking practical elements into consideration.
Recently, a light emitting element using an orthometalated complex of iridium (Ir(ppy)3: tris-orthoiridiated complex with 2-phenylpyridine) as a light emitting material has been reported as a light emitting element which can emit with higher efficiency (Applied Physics Letters, vol. 75, page 4, 1999). The external quantum efficiency of this light emitting element is 8.3%, which exceeds 5%, which has been said to be the limit of levels of external quantum efficiency. The above orthometalated complex of iridium is limited to a green color light emitting element. Therefore, it is necessary to also develop elements which emit other color lights with high efficiency when the elements are applied in full color displays or white light emitting elements.
On the other hand, among organic light emitting elements, those which have attained high illuminance emission are elements with multilayered structure of organic materials by vacuum deposition. Production of elements by using a coating method is preferable in view of simplification of production steps, processability and increase in area. However, elements produced by a conventional coating method are inferior to elements produced by a deposition system in emission illuminance and luminance efficiency, giving rise to important problems concerning high illuminance and high luminance efficiency in view of the simplification of production steps, processability, area increase and the like.